By combining commonly used transfection agents (TA) that have high net positive electrostatic charge with Superparamagnetic dextran coated iron oxide nanoparticle (SPIO) such as the FDA approved agent Ferumoxides (FE), a complex is formed that can be used to magnetically label stem cells and other mammalian cells. Protamine Sulfate (Pro) is an FDA approved drug that is used to treat heparin anticoagulation was found to be a more efficient transfection agent and when complexed to FE in labeling cells. Protamine sulfate (Pro) is now used to magnetically label cells. No adverse effect on the short or long-term toxicity, cell viability and functional capacity or differentiation capacity of stem cells was observed following magnetic cell labeling with FE-Pro. Systemically administered Fe-Pro labeled hematopoietic CD34 AC 133 cells can be detected by in vivo magnetic resonance imaging (MRI) in a mouse tumor model. MRI was performed during tumor growth. Mice that received labeled cells demonstrated hypointense regions within the tumor that evolved over time . Histology showed iron-labeled cells around the tumor rim in labeled mice, which expressed CD31 and von Willebrand factor, indicating the transplanted cells detected in the tumor have differentiated into endothelial-like cells. These results demonstrate that MRI can detect the incorporation of magnetically labeled bone marrow-derived precursor cells into tumor vasculature as part of ongoing angiogenesis and neovascularization. Gadolinium-fullerenol (Gd@C82) is a paramagnetic agent that has unique NRM relaxation time properties was evaluated as a possible intracellular contrast agent to label cells. Protamine sulfate transfection increased cell uptake of Gd@C82 fullerenols. The label was distributed in endosomes in the cytoplasm as shown by electron microscopy. High viability was shown for all cell lines and normal differentiation capacity was shown for mesenchymal stem cells (MSC) although there was some effect of Gd@C82 fullerenol on cell proliferation. Injections of labeled cells into muscle could be detected as hyperintense region compared to surrounding tissue on MRI. This study suggests that further investigation of Gd fullerenols for tracking studies of viable cells, including stem cells, is warranted.